A device controller 100 serving as a conventional memory card control device is shown in FIG. 4. The device controller 100 includes an interface controller 101, a CPU 102, a ROM 103, an internal PLL controller 104, an internal PLL 105, and a memory card controller 106.
The interface controller 101 communicates with a host computer 200 which is externally connected to the device controller 100 to receive data to be written to a memory card and send data read from the memory card.
The CPU 102 controls the interface controller 101 and the memory card controller 106 which is described later. The CPU 102 manages overall control of the device controller 100. The ROM 103 is a read only memory in which software for controlling the CPU 102 is stored in advance.
The internal PLL controller 104 controls the internal PLL 105 which is described later. The internal PLL 105 supplies clock signals in a predetermined frequency to the interface controller 101 and the memory card controller 106.
The memory card controller 106 includes plural card controllers 1061, 1062, 1063, . . . , and 106n, each of which controls writing, reading, and the like of data to/from the memory card inserted in a memory card insertion unit.
The device controller 100 as shown in FIG. 4 is connected to, for example, the host computer 200 through a USB (Universal Serial Bus) interface and controls writing and reading to/from various memory cards to serve as an external memory device of the host computer 200.
The device controller 100 of this kind is sometimes connected to or mounted in a notebook computer serving as a host computer. In such cases, power consumption is an issue.
In a battery-powered notebook computer, power consumption directly affects battery life. In a recent memory card, in particular, capacity has been increasing, a processed data amount has increased to a G (giga) unit, and a data transfer rate has been also increasing. Therefore, there is a concern that more power consumption is expected.
Patent Document 1 discloses a method to solve such a problem. In this method, when a computer detects that a device connected to the computer is in an idle state, the computer sends a function to cause the device to be in a suspended state (in a low power consumption state).    [Patent Document 1] Japanese Patent Application Publication No. 2005-508041
However, since the host computer is required to monitor the state of the device, a monitoring function is required to be added to an OS (operating system), a device driver, or the like on the host computer side.